Semiconductor device and its manufacturing method

ABSTRACT

In a semiconductor device including a memory cell array formed of memory cells using a storage element by a variable resistor and a select transistor, a buffer cell is arranged between a sense amplifier and the memory cell array and between a word driver and the memory cell array. The resistive storage element in the memory cell is connected to a bit-line via a contact formed above the resistive storage element. Meanwhile, in the buffer cell, the contact is not formed above the resistive storage element, and a state of being covered with an insulator is kept upon processing the contact in the memory cell. By such a processing method, exposure and sublimation of a chalcogenide film used in the resistive storage element can be avoided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 12/672,685filed Feb. 8, 2010, Now, U.S. Pat. No. 8,179,739 which is a 371 ofInternational Patent Application No. PCT/JP2007/065686 filed Aug. 10,2007.

TECHNICAL FIELD

The present invention relates to a semiconductor device and a method ofmanufacturing the same, and to a technique effectively applied to astorage device including a memory cell formed of a device having aresistance value that varies corresponding to storage information, inparticular, a storage device including a phase change memory using amemory cell which stores information utilizing a state change of achalcogenide material and detects a resistance value difference from theinformation to discriminate the information.

BACKGROUND ART

As a technique which has been studied by the present inventors, forexample, the following technique has been considered regarding asemiconductor device including a phase change memory.

A storage element uses a chalcogenide material (or phase changematerial) such as Ge—Sb—Te-based or Ag—In—Sb—Te-based one containing atleast antimony (Sb) and tellurium (Te) as materials for a storage layer.The characteristics of the phase change memory using the chalcogenidematerial are described, for example, in Non-Patent Document 1.

FIG. 2 is a graph illustrating a relationship between a pulse width anda temperature which are required for phase change of a resistive storageelement using a phase change material. When storage information “0” iswritten in the resistive storage element, such a reset pulse as to heatthe device up to a melting point Ta of a chalcogenide material or higherand to rapidly cool the same is applied to the device, as illustrated inFIG. 2. By setting a cooling time t1 short, for example, to about 1 ns,the chalcogenide material is changed to a high-resistance amorphous(noncrystalline) state.

On the contrary, when writing of storage information “1” is performed,the chalcogenide material is changed to a poly crystal state having alow resistance by applying such a set pulse as to hold the temperatureof the resistive storage element in a temperature range lower than themelting point Ta and higher than a crystallization temperature Tx thatis higher than or equal to a glass-transition temperature. A timerequired for crystallization t2 varies depending on the composition ofthe chalcogenide material. The temperature of the device illustrated inFIG. 2 depends on Joule heat which the resistive storage element itselfgenerates and on thermal diffusion to the environment.

A typical phase change memory includes a memory-cell array MCA, a blockof word drivers WDB, a multiplexer MUX, a program circuit PRGM, and asense amplifier SA, as illustrated in FIG. 3. The memory-cell array MCAincludes memory cells MC00, MC10, . . . arranged in matrix atintersection points of word lines WL0, WL1, . . . and bit-lines BL0,BL1, . . . . The memory cell has a configuration in which theabovementioned resistive storage element RE and a select transistor CTare inserted between the bit-line BL0 and a ground terminal, forexample, as illustrated in MC00. A gate electrode of the selecttransistor CT is connected to the word line WL0. The block of worddrivers WDB selects one from the word lines WL0, WL1, . . . in responseto an address signal (not illustrated). The multiplexer MUX selects onefrom the bit-lines BL0, BL1, . . . in response to an address signal (notillustrated) and connects the bit-line to the program circuit PRGM orthe sense amplifier SA.

In Patent Document 1, a layout structure of a semiconductor memorydevice having a hierarchical structure and a layout method thereof aredescribed. Specifically, the same structure as that of the memory cellis formed on a wiring area for a global bit-line and a regularity of alayout pattern of a structure in a memory-cell array is maintained. InPatent Document 2, such disposing a structure similar to that of thememory cell around a memory-cell array is described.

-   Patent Document 1: Japanese Patent Application Laid-Open Publication    No. 2006-295117-   Patent Document 2: Japanese Patent Application Laid-Open Publication    No. 2004-349504-   Patent Document 3: WO2005/117118-   Non-Patent Document 1: “IEEE International Electron Devices meeting,    TECHNICAL DIGEST,” (USA), 2001, pp. 803-806-   Non-Patent Document 2: “IEEE International Solid-State Circuits    Conference, Digest of Technical Papers,” (USA), 2007, pp. 472-473-   Non-Patent Document 3: “IEEE International Solid-State Circuits    Conference, Digest of Technical Papers,” (USA), 2007, pp. 474-475

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Now, as a result of a study on the technology of the phase change memoryas described above made by the present inventors, it has become apparentthat it is necessary to take measures for preventing contamination of amanufacturing apparatus due to sublimation of the chalcogenide materialupon forming a resistive storage element.

First, a layout of the phase change memory has been studied. FIG. 4Aillustrates a circuit configuration diagram of memory cells on abit-line BL0 in the memory-cell array illustrated in FIG. 3, FIG. 4Billustrates a layout of the memory cells, and FIG. 4C illustrates across-section structure of the memory cells. In the layout diagramillustrated in FIG. 4B, AA0, AA1, AA2, . . . are active area patternsforming a drain electrode, a source electrode, and a channel of a selecttransistor. The active area pattern is divided by two memory cells. Forexample, the active area pattern AA0 is used for forming the memorycells MC00 and MC10. FGs are polysilicon patterns forming the word linesWL0, WL1, . . . RLA and RL are resistive storage element patterns.Especially, the pattern RLA is for a storage element in the memory cellMC00 arranged at an end of the bit-line BL0. BC is a bottom contactpattern for connecting an active area and a resistive storage element.TC and TCA are top contact patterns for connecting a resistive storageelement and an upper wiring layer (not illustrated). Especially, thepattern TCA is a top contact formed on the resistive storage element inthe memory cell MC00 arranged at the end of the bit-line BL0.

The cross-sectional view of FIG. 4C illustrates a structure of a mainpart for ease of description. 100 denotes a p-type silicon substrate.101 denotes a word line connected with a gate electrode of a selecttransistor. 102 denotes an n+ diffusion layer to be a drain electrodeand a source electrode of a select transistor. 104 denotes a P-well and106 denotes an insulator for isolation. 120 and 120A denote resistivestorage elements. Especially, 120A denotes a resistive storage elementin the memory cell MC00. 131 denotes a bottom contact. 132 and 132Adenote top contacts. Especially, 132A denotes a top contact formed onthe resistive storage element 120A in the memory cell MC00 arranged atthe end of the bit-line BL0.

In such a structure, since a pattern density of the resistive storageelements lowers in the memory cell MC00 arranged at the end of thebit-line BL0, namely, near an outer periphery of the memory-cell array,patterning may be performed to have an area smaller than an originaldesign size in a photolithography process. Further, in a dry etchingprocess performed thereafter, etching in a lateral direction advancestoo much due to a loading effect caused by the lowering of the patterndensity, which may result in reduction of an area of the final resistivestorage element. It has been found that, when the area of the storageelement becomes relatively small in this manner, a top contact patternTCA (132A) may protrude from a storage element pattern RLA (120). Across-section structure regarding a resistive storage element isdescribed in detail with reference to FIG. 5 to consider the problem.

In fact, the resistive storage elements 120 and 120A (RL, RLA) describedin FIGS. 4A-4C each includes an interfacial layer 300, a chalcogenidematerial 301, and a tungsten electrode 302, as illustrated in FIG. 5.Further, the storage element is protected by silicon nitrides 303 and304. 400 denotes an Interlayer Dielectric film. Top contact holes 132Hand 132AH are contact holes in which tungsten forming the top contacts132 and 132A are buried. These contact holes are formed on the bottomcontact 131 using the silicon nitride 304 as an etch-stopper and on theresistive storage element using the silicon nitride 303 as anetch-stopper. In the memory cell MC00, however, the top contact 132AHprotrudes from the storage element, so that the silicon nitride 304formed on a sidewall of the storage element is scraped off. Therefore,burying of tungsten serving as the top contact is performed having thechalcogenide material 301 being exposed using a vacuum CVD (ChemicalVapor Deposition) apparatus. In this process, since a target wafer isheated to a sublimation temperature (about 200° C.) of the chalcogenidematerial or higher, the CVD apparatus may be contaminated by sublimationof the chalcogenide material. When areas of the storage element patternsRL and RLA are increased such that the top contact pattern TCA (132A)does not protrude in order to avoid this problem, such a problem that anarea of the memory-cell array increases newly occurs.

Second, the depth of the dry etching of the abovementioned top contacthole has been studied. FIG. 6 illustrates a cross-section structureobtained when the pattern of the resistive storage element (120, RL) inthe memory cell MC00 has been processed to a desired size. A point topay attention in FIG. 6 is that the Interlayer Dielectric film 400becomes thin in the memory cell MC00 arranged at the end of the bit-lineBL0, namely, at an outer periphery of the memory-cell array due to CMP(Chemical Mechanical Polishing). As a result, a top contact hole 132HBmay break through the silicon nitrides 304 and 303 and the tungstenlayer 302 to reach the chalcogenide 301. As previously described in FIG.5, tungsten configuring the top contact is deposited on the exposedchalcogenide material 301 even in such a situation. Thereby, when thetarget wafer is heated in this process, the inside of the CVD apparatusmay be contaminated due to sublimation of the chalcogenide material.

A subject of the present invention is to solve these problems. That is,a preferred aim of the present invention is to achieve a phase changememory having a small area without exposure of a chalcogenide materialdue to unevenness in a memory array processing.

The above and other preferred aims and novel characteristics of thepresent invention will be apparent from the description of the presentspecification and the accompanying drawings.

Means for Solving the Problems

The typical ones of the inventions disclosed in the present applicationwill be briefly described as follows.

That is, in a semiconductor device including a phase change memory,buffer cells are arranged between a sense amplifier and a memory-cellarray and between a word driver and the memory cell array. The buffercell is configured by a resistive storage element and a selecttransistor same with those in a memory cell. The resistive storageelement in the memory cell is connected to a bit-line via a contactformed on the resistive storage element. On the other hand, in thebuffer cell, no contact is formed on the resistive storage element andwhen a contact within the memory cell is processed, the resistivestorage element is maintained in a state of being covered with aninsulating material. Exposure or sublimation of a chalcogenide film usedin the resistive storage element is avoided by such a processing method.

Effects of the Invention

According to the present invention, a phase change memory can bemanufactured without resulting in contamination of a manufacturingapparatus.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIGS. 1A-1B are diagrams illustrating an example of a layout and across-section structure of a phase change memory included in asemiconductor device of a first embodiment of the present invention;

FIG. 2 is a graph illustrating a relationship between a pulse width anda temperature required for phase change of a resistance device using aphase change material;

FIG. 3 is a diagram illustrating a configuration example of a phasechange memory;

FIGS. 4A-4C are diagrams illustrating an example of a layout and across-section structure of the phase change memory illustrated in FIG.3;

FIG. 5 is a diagram illustrating an example of a cross-section structureof the phase change memory illustrated in FIGS. 4A-4C in detail;

FIG. 6 is a diagram illustrating another example of a cross-sectionstructure of the phase change memory illustrated in FIGS. 4A-4C indetail;

FIG. 7 is a diagram illustrating an example of a circuit configurationof the phase change memory of FIGS. 1A-1B;

FIG. 8 is a diagram illustrating an example of a photolithographyprocess for the phase change memory illustrated in FIGS. 1A-1B indetail;

FIG. 9 is a diagram illustrating an example of a cross-section structureof the phase change memory illustrated in FIGS. 1A-1B in detail;

FIGS. 10A-10B are diagrams illustrating an example of a layout and across-section structure of a phase change memory included in asemiconductor device of a second embodiment of the present invention;

FIG. 11 is a diagram illustrating an example of a circuit configurationof a phase change memory included in a semiconductor device of a thirdembodiment;

FIGS. 12A-12B are diagrams illustrating an example of a layout and across-section structure of the phase change memory illustrated in FIG.11;

FIG. 13 is a diagram illustrating an example of a cross-sectionstructure of the phase change memory illustrated in FIGS. 12A-12B indetail;

FIGS. 14A-14B are diagrams illustrating an example of a layout and across-section structure of a phase change memory included in asemiconductor device of a fourth embodiment of the present invention;

FIG. 15 is a diagram illustrating an example of a circuit configurationof a phase change memory included in a semiconductor device of a fifthembodiment;

FIGS. 16A-16B are diagrams illustrating an example of a layout and across-section structure of the phase change memory illustrated in FIG.15;

FIG. 17 is a diagram illustrating an example of a circuit configurationof a phase change memory included in a semiconductor device of a sixthembodiment;

FIGS. 18A-18B are diagrams illustrating an example of a layout and across-section structure of the phase change memory illustrated in FIG.17;

FIG. 19 is a diagram illustrating an example of a circuit configurationof a phase change memory in a semiconductor device of a seventhembodiment;

FIGS. 20A-20B are diagrams illustrating an example of a layout and across-section structure of the phase change memory illustrated in FIG.19;

FIGS. 21A-21B are diagrams illustrating another example of a layout anda cross-section structure of the phase change memory illustrated in FIG.19;

FIG. 22 is a diagram illustrating an example of a circuit configurationof a phase change memory included in a semiconductor device of an eighthembodiment;

FIGS. 23A-23 F are diagrams illustrating examples of timing diagrams ofset operations of the phase change memory illustrated in FIG. 22;

FIG. 24 is a diagram illustrating an example of a circuit configurationof a phase change memory included in a semiconductor device of a ninthembodiment;

FIGS. 25A-25C are diagrams illustrating another example of a layout anda cross-section structure of the phase change memory illustrated in FIG.24;

FIG. 26 is a diagram illustrating an example of a circuit configurationof a phase change memory included in a semiconductor device of a tenthembodiment; and

FIGS. 27A-27C are diagrams illustrating another example of a layout anda cross-section structure of the phase change memory illustrated in FIG.26.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that componentshaving the same function are denoted by the same reference symbolsthroughout the drawings for describing the embodiment, and therepetitive description thereof will be omitted. Further, a circuitdevice configuring each block in the embodiments is not limited to aspecific one, but it is typically formed on one semiconductor substratesuch as single crystal silicon by a technology of integrated circuitfor, for example, a known CMOS (complementary MOS transistor). Further,a chalcogenide material or the like exhibiting phase change is formed ina hybrid manner with a manufacturing technology of an integratedcircuit.

First Embodiment

FIG. 7 illustrates a configuration example of a main block of a phasechange memory according to a first embodiment of the present invention.That is, the phase change memory includes a memory-cell array MCA andeight buffer-cell arrays YLBCA, YRBCA, XUBCA, XBBCA, ULBCA, URBCA,BLBCA, and BRBCA arranged around the memory-cell array MCA in additionto a multiplexer MUX, a block of word drivers WDB, a program circuitPRGM, and a sense amplifier SA. A configuration excepting the eightbuffer-cell arrays is the same as the configuration illustrated in FIG.3. A feature of the phase change memory lies in that a cell including aresistive storage element RE disconnected from a bit-line BL0 and aselect transistor CT, for example, such as a buffer cell YBC00 in thebuffer-cell array YLBCA is arranged around the memory-cell array MCA. Aconfiguration of the buffer-cell array will be described below indetail.

The buffer-cell array YLBCA (a second plurality of buffer cells)includes buffer cells YBC00, YBC10, . . . arranged at respectiveintersection points of two ground voltage supply lines (second voltagesupply lines) and bit lines BL0, BL1, . . . , and it is arranged betweenthe multiplexer MUX and the memory-cell array MCA. The resistive storageelement RE and the select transistor CT configuring each of the buffercells have the same structure as that of the memory cell in thememory-cell array MCA. The buffer-cell array YRBCA has the sameconfiguration as that of the buffer-cell array YLBCA and it is disposedat the other end of each bit-line so as to be paired with thebuffer-cell array YLBCA.

The buffer-cell array XUBCA (a first plurality of buffer cells) includesbuffer cells XBC00, XBC10, . . . arranged at respective intersectionpoints of two ground voltage supply lines (first voltage supply lines)and word lines WL0, WL1, . . . , and it is arranged between the block ofword drivers WDB and the memory-cell array MCA. The two ground voltagesupply lines correspond to bit-lines in the memory-cell array MCA, andthe resistive storage element in each buffer cell is disconnected fromthe ground voltage supply lines. The buffer-cell array XBBCA has thesame configuration as that of the buffer-cell array XUBCA, and it isdisposed at the other end of the each word line so as to be paired withthe buffer-cell array XUBCA.

Each of the buffer-cell arrays ULBCA, URBCA, BLBCA, and BRBCA includesbuffer cells CBC00, CBC10, CBC01, and CBC11 arranged at intersectionpoints of two ground voltage supply lines parallel to the word lines andtwo ground voltage supply lines parallel to the bit-lines. Thesebuffer-cell arrays are arranged around the memory-cell array MCAtogether with the other buffer-cell arrays YLBCA, YRBCA, XUBCA, andXBBCA.

FIG. 1A illustrates a layout of the buffer-cell array YLBCA and thememory-cell array MCA on the bit-line BL0 of the phase change memoryillustrated in FIG. 7, and FIG. 1B illustrates a cross-section structureof the same. As compared with FIG. 4, a pattern FM of a first metallayer 110, a pattern SM of a second metal layer 111, and a pattern FV ofa first via 130 are newly added. The first metal layer 110 is used asthe ground voltage supply line. The second metal layer 111 is used asthe bit-line BL0. The first via 130 is disposed on the same axis as theaxis of the top contact (TC), and it is used for connecting theabovementioned first metal layer 110 and second metal layer 111 to eachother. In the buffer cells YBC00 and YBC10, the connection between theresistive storage element (RL, 120) and the bit-line (SM, 111) is cut byremoving the top contact (TC, 132) on the resistive storage element (RL,120), as illustrated in the circuit diagram in FIG. 7.

FIG. 8 illustrates an example of a photolithography process as aprocessing method for forming the top contact. A resist 500 beingapplied on an upper portion of an Interlayer Dielectric film 400 isexposed using a photomask 600 having a photo-shielding film 602 beingremoved in the same pattern as that of the top contact on a glass plate601. Next, a resist area 501 being exposed by exposure light 700 isremoved by using a developer. Further, when an etching processing isperformed using a resist on the remaining portions which have not beenexposed as a mask, top contact holes 132H are formed at desiredpositions, as illustrated in FIG. 9.

By adopting such a structure as described above, the sublimation problemof chalcogenide and the contamination problem of the manufacturingapparatus in the processing of the phase change memory can be avoided.That is, the resistive storage element in the buffer cells YBC00 andYBC10 are maintained in a state of being protected by the InterlayerDielectric film 400 at a formation time of the top contact holes 132H,as illustrated in FIG. 8. Therefore, there is not a concern thatexposure or sublimation of the chalcogenide film 301 occurs even whenthe pattern of the resistive storage element is small or the InterlayerDielectric film on the buffer-cell array YLBCA is thin. Accordingly, itbecomes possible to prevent contamination of the manufacturing apparatusand production throughput is improved. Furthermore, in the memory-cellarray MCA, since the area of the resistive storage element pattern RLcan be suppressed, a phase change memory having a small area can beformed. Moreover, since the resistive storage elements RE and the selecttransistors CT are arranged regularly, shape variation in thememory-cell array MCA can be suppressed, so that a phase change memoryhaving even electric characteristics can be achieved.

Second Embodiment

FIGS. 10A and 10B illustrate another example of a layout and across-section structure of a phase change memory according to a secondembodiment of the present invention. A difference of this embodimentfrom the first embodiment illustrated in FIGS. 1A-1B in layout lies inthat the top contacts TC are further arranged at the end of the bit-lineBL0, namely, outside the buffer-cell array YLBCA. The top contacts areformed at the same positions as the case of arranging the memory cellsin this area.

With such a configuration, the top contacts of the memory cell (here,the memory cell MC00) positioned at the outermost periphery in thememory-cell array MCA are positioned inside the top contacts arranged inmatrix. Thereby, the top contacts of the memory cell MC00 are formed inan area where the density of the top contacts is substantially constant.Accordingly, shape variation in the memory-cell array MCA is furthersuppressed, so that a phase change memory having further even electriccharacteristics can be achieved.

Third Embodiment

In a third embodiment of the present invention, a configuration exampleof a main block of a phase change memory different from thepreviously-described invention will be described. A feature of thisinvention lies in a configuration in which the resistive storage elementis removed from the buffer cell. The feature will be described belowwith reference to FIG. 11 to FIG. 13.

FIG. 11 illustrates a configuration example of a main block of a phasechange memory according to the third embodiment. Eight buffer-cellarrays YLBCA, YRBCA, XUBCA, XBBCA, ULBCA, URBCA, BLBCA, and BRBCA arearranged around a memory-cell array MCA in the same manner as FIG. 7.These buffer-cell arrays include buffer cells having a select transistorCT. A source electrode and a gate electrode of the select transistor CTare connected to ground voltage supply lines, respectively.

FIG. 12A illustrates a layout of the buffer-cell array YLBCA and thememory-cell array MCA on the bit-line BL0 of the phase change memoryillustrated in FIG. 11, and FIG. 12B illustrates a cross-sectionstructure of the same. As compared with FIG. 1, the present embodimentis different from FIGS. 1A-1B in that the resistive storage element (RL,120) is removed in the buffer cells YBC00 and YBC10. Instead, the topcontact (TC, 132) on the resistive storage element (RL, 120) is formedlike the memory cell MC00 and the like. The top contacts are formedusing a photolithography process such as illustrated in FIG. 8 of thefirst embodiment.

By adopting such a structure, such an effect as avoidance of thesublimation problem of chalcogenide and the contamination problem of themanufacturing apparatus in the processing of the phase change memory canbe obtained like the previous embodiments. That is, top contact holes132H are formed at positions at which the resistive storage element 120does not exist in the buffer cells YBC00 and YBC10, as illustrated inFIG. 13. Therefore, even when misalignment occurs between the resistivestorage element 120 and the top contact hole 132H or even when anInterlayer Dielectric film on the buffer-cell array YLBCA is thin, thereis not a concern about exposure or sublimation of the chalcogenide film301. Thereby, it becomes possible to prevent contamination of themanufacturing apparatus so that the production throughput is improved.Further, since the top contacts are arranged regularly, shape variationin the memory-cell array MCA can be suppressed, so that a phase changememory having even electric characteristics can be achieved.

Fourth Embodiment

FIGS. 14A and 14B illustrate another example of a layout and across-section structure of a phase change memory according to a fourthembodiment of the present invention. A difference of this embodimentfrom the third embodiment illustrated in FIGS. 12A-12B in layout lies inthat the resistive storage elements RL are further arranged at the endof the bit-line BL0, namely, outside the buffer-cell array YLBCA. Theresistive storage elements are formed at the same positions as the caseof arranging the memory cells in this area.

With such a configuration, the resistive storage elements RL of thememory cell (here, the memory cell MC00) positioned at the outermostperiphery in the memory-cell array MCA are positioned inside theresistive storage elements RL arranged in matrix. Therefore, theresistive storage elements RL of the memory cell MC00 are formed in anarea where the density of the resistive storage elements RL issubstantially constant. Accordingly, shape variation in the memory-cellarray MCA is further suppressed, so that a phase change memory havingfurther even electric characteristics can be achieved.

Fifth Embodiment

Since a programming operation of storage information is performed in thephase change memory by Joule heat generated in the resistive storageelement, it is important to cause a large current to flow in a memorycell while suppressing the wire resistance as much as possible to reducea voltage drop in the wiring resistance. In a fifth embodiment of thepresent invention, means for wiring ground voltage supply lines in agrid-like manner is provided in order to suppress the wiring resistanceon the side of the source of a memory cell. That is, a configurationexample of a source shunt cell will be described.

FIG. 15 illustrates a diagram of a main block of a phase change memoryaccording to the present invention. In FIG. 15, a configuration examplewhere the memory-cell array MCA is divided into two memory-cell arraysMCAU and MCAB and a source shunt cell array SSA is disposed therebetweenis illustrated for ease of explanation. The memory cell array MCAUincludes memory cells MC00, MC10, . . . , MC70 arranged at respectiveintersection points of word lines WL0 to WL7 and bit-lines BL0 to BL3 inan array of 8 rows and 4 columns. In the same manner, the memory cellarray MCAB includes memory cells MC00, MC10, . . . , MC70 arranged atrespective intersection points of the word lines WL0 to WL7 andbit-lines BL4 to BL7 in an array of 8 rows and 4 columns. The sourceshunt cell array SSA includes eight source shunt cells SC0 to SC7arranged at respective intersection points of the word lines WL0 to WL7and a ground voltage supply line. Each of the source shunt cellsincludes a select transistor CT similar to that in the memory cell.Incidentally, the number of illustrated bit-lines or word lines is eightfor ease of description, but the number is not limited to this.

FIG. 16A illustrates a layout of the source shunt cell array SSA of thephase change memory illustrated in FIG. 15 and FIG. 16B illustrates across-section structure thereof. As compared with FIG. 1, the presentembodiment is different in that the resistive storage element (RL, 120)is removed from the memory cell. Further, a first metal layer (FM) and afirst via (FV, 130) are further formed between a bottom contact (BC,131) and a top contact (TC, 132) which are formed on an n+ diffusionlayer (102) so as to be shared by two select transistors and a secondmetal layer (SM, VSS) arranged in parallel with a bit-line.

By adopting such a structure, it is made possible to arrange groundvoltage supply lines in a grid manner in a memory array using the firstmetal layer FM and the second metal layer SM, so that the wiringresistance on the source side can be reduced. By configuring the sourceshunt cell to have a select transistor same as that in the memory cell,arrangement of the select transistors becomes regular, so that shapevariation in the memory cell can be suppressed. Thereby, even when asource shunt cell is arranged within a memory array, electriccharacteristic variation of the memory cell can be suppressed, so that aphase change memory operating stably can be achieved.

Sixth Embodiment

In order to achieve a high-speed operation in a phase change memoryhaving a large capacity, it is important to reduce a resistance of aword line to shorten a rising time upon activation of the word line. Ina sixth embodiment of the present invention, means for short-circuitinga metal wiring layer arranged in parallel with a gate electrode ofpolysilicon at a fixed interval is provided in order to reduce thewiring resistance of word lines in the memory cell. That is, aconfiguration example of a word-line-shunt cell will be described.

FIG. 17 illustrates a diagram of a main block of a phase change memoryaccording to the sixth embodiment. In FIG. 15, a configuration examplein which the memory-cell array MCA is divided into two memory-cellarrays MCAU and MCAB and a word-line-shunt cell array WSA is disposedtherebetween is illustrated for ease of description. The memory cellarray MCAU includes memory cells MC00, MC10, . . . , MC70 arranged atrespective intersection points of word lines WL0 to WL7 and bit-linesBL0 to BL3 in an array of 8 rows and 4 columns. In the same manner, thememory cell array MCAB includes memory cells MC00, MC10, . . . , MC70arranged at respective intersection points of the word lines WL0 to WL7and bit-lines BL4 to BL7 in an array of 8 rows and 4 columns. Theword-line-shunt cell array WSA is configured by eight word-line-shuntcells WC0 to WC7 connecting the word lines WL0 to WL7 and global wordlines GWL0 to GWL7 arranged in parallel with the respective word lines.

FIG. 18A illustrates a layout of the word-line-shunt cell array WSA ofthe phase change memory illustrated in FIG. 17, and FIG. 18B illustratesa cross-section structure of the same. As compared with the structureillustrated in FIG. 1, the resistive storage element (RL, 120), theactive areas (AA0, AA1, . . . ), and the n+ diffusion layer (102) areremoved from the memory cell, and word line shunt cells WC0, WC1, . . ., WC7 are formed on the insulator 106 for isolation. Further, the wordline formed of polysilicon forms a convex-type layout pattern in an areawhere bottom contacts are formed. The word line is connected to a globalword line formed of a third metal layer (TM, 112) via the top contact(TC, 132), the first metal layer (FM, 110), the via (FV, 130), thesecond metal layer (SM, 111), and the second via (SV, 133) which areformed on the same axis as that of the bottom contact (BC, 131).

By adopting such a structure, it is made possible to connect the wordline formed of polysilicon and the global word line formed of the thirdmetal layer to each other within the memory array, so that the wiringresistance of the word line can be reduced. Further, since formation canbe made with the same area as that of the memory cell, area overhead ofthe memory-cell array can be suppressed, and shape variation of thememory cell can be suppressed by suppressing an area where the layoutpatterns of the memory cells become discontinuous. Thereby, a high-speedphase change memory having a small area and reduced electriccharacteristic variation can be achieved.

Seventh Embodiment

In a seventh embodiment of the present invention, a method to achieve areference cell used for generation of a reference signal in a readoperation of a phase change memory will be described. FIG. 19illustrates a block diagram of a main portion of a phase change memoryaccording to the seventh embodiment of the present invention. A featureof the seventh embodiment lies in that a reference cell is disposed oneach bit-line in each of the memory-cell arrays MCAL and MCAR. Withpaying attentions to this point, a configuration of the phase changememory according to the seventh embodiment will be described in detailbelow.

The phase change memory illustrated in FIG. 19 includes a programcircuit PRGM, a sense amplifier SA, a read/write circuit select circuitRWSEL, memory-cell arrays MCAL and MCAR, and multiplexers MUXL and MUXR.The read/write circuit select circuit RWSEL is a circuit block whichconnects one of the program circuit PRGM or the sense amplifier SA to aselected bit line via a common data line CDLL or CDLR and themultiplexer MUXL or MUXR. The multiplexer MUXL is a circuit block whichselects one from the bit lines BL0L to BL7L in the memory-cell arrayMCAL to connect the selected one to the common data line CDLL. In thesame manner, the multiplexer MUXR is a circuit block which selects onefrom the bit-lines BL0R to BL7R in the memory-cell array MCAR to connectthe selected one to the common data line CDLR.

The memory-cell arrays MCAL and MCAR include a block of word driversWDB, reference cell arrays RCAU, RCAB, and a reference cell source-shuntcell RSC in addition to the memory-cell arrays MCAU, MCAB and thesource-shunt cell array SSA illustrated in FIG. 15. Among them, thereference cell array RCAU includes reference cells RC0 to RC3 arrangedat intersection points of the bit-lines BL0R to BL3R and the referenceword line RWL. Similarly, the reference cell array RCAB includesreference cells RC0 to RC3 arranged at intersection points of thebit-lines BL4R to BL7R and the reference word line RWL. Each of thereference cells RC0 to RC3 is formed of NMOS transistors RT and CT in acascade connection between the bit-line BL0R and the ground voltagesupply line, for example, like the reference cell RC0. The transistor CThas the same configuration as that of the select transistor in thememory cell. The reference word lines are selectively activatedaccording to the memory cell selected. The transistor RT is designedsuch that its gate length LRT is longer than a gate length LCT of theselect transistor CT. Further, a bias voltage VBIAS inputted into thegate electrode is controlled by a power source circuit (notillustrated). A feeder line of the bias voltage VBIAS is arranged inparallel with the reference word line RWL. With such a configuration, adriving current to the reference cell can be optimized so as to generatea desired reference signal.

The reference cell source shunt cell RSC is arranged at an intersectionpoint of the ground voltage supply line and the reference word line RWLand it includes transistors RT and CT like the reference cell. Adifference of the reference cell source shunt cell RSC from thereference cell lies in that a source terminal is connected to both aground voltage supply line parallel to the reference word line RWL and aground voltage supply line parallel to the bit line. Such connection canbe easily understood from a layout and a cross-sectional view describedlater.

FIG. 20A illustrates a layout of the memory cells MC00 to MC70 and thereference cell RC0 in the bit-line BL0R, and FIG. 20B illustrates across-section structure of the same. The reference cell RC0 is formed onan activation area AA4 having the same area as that of activation areasAA0 to AA3 in which the memory cells MC00 to MC70 are formed. The sourceelectrode of the transistor CT and the ground voltage supply lineparallel with the reference word line are connected to each other byconnecting an n+ diffusion layer 102 in the source electrode of thetransistor CT to the convex-pattern first metal layer (FM, 110) via thebottom contact (BC, 131) and the top contact (TC, 132). The drainelectrode of the transistor RT and the bit line BLR0 are connected toeach other by connecting the n+ diffusion layer 102 in the drainelectrode of the transistor RT to the second metal layer (SM, 111) viathe bottom contact (BC, 131) and the top contact (TC, 132), and thefirst metal layer (FM, 110) and the first via (FV, 130) which are formedon the same axis.

FIG. 21A illustrates a layout of a source-shunt cell array SSA and areference cell source-shunt cell RSC and FIG. 21B illustrates across-section structure thereof. The reference cell source-shunt cellRSC is based upon the structure of the abovementioned reference cellsRC0 to RC3. Further, the source electrode of the transistor CT and theground voltage supply line arranged in parallel with the bit-line areconnected to each other by connecting the first metal layer and thesecond metal layer via the first via (FV, 130).

By adopting such a reference cell structure, it is made possible to formthe reference cells at the same pitch as that of the memory cells in thememory-cell array, and it is made possible to suppress shape variationsof the memory cell and the reference cell. Thereby, it is possible toform a memory-cell array with a small area and reduced electriccharacteristic variation. The wiring resistance of the source line inthe reference cell can be reduced by using the reference cellsource-shunt cell RSC. Further, a read operation of a configurationcalled open-type bit line configuration can be performed by causing twomemory-cell arrays MCAL and MCAR to share the program circuit PRGM andthe sense amplifier SA and using one of the memory-cell arrays for readwhile using the other for reference signal generation.

Eighth Embodiment

In an eighth embodiment of the present invention, another programmingoperation of memory cells and memory-cell arrays of a phase changememory will be described. A feature of the eighth embodiment of thepresent invention lies in that an activation time of a word line ischanged according to an operation in a memory-cell array in which eachmemory cell has a 2T1R configuration (two transistors and one resistivestorage element), as described in Patent Document 3.

FIG. 22 illustrates a memory cell array TMCA in a phase change memoryaccording to the present invention. In FIG. 22, an example in which thememory cell array TMCA includes memory cells MC00 to MC77 arranged in 8rows and 8 columns is described for simplification. The memory cellincludes NMOS transistors CT0 and CT1 and a resistive storage elementRE, for example, as illustrated in MC00. The resistive storage elementRE is inserted between the bit-line BL0 and two transistors CT0 and CT1.The two transistors CT0 and CT1 are controlled by word lines WL00 andWL01, respectively. Further, source electrodes of the transistors CT0and CT1 are connected to source electrodes of transistors of adjacentmemory cells.

FIGS. 23A-23F illustrate operations of the memory cell according to theeighth embodiment. In FIG. 23, operation voltage waveforms of the wordlines WL00 and WL01 and the bit-line BL0 are illustrated with payingattention to the memory cell MC00. FIG. 23A illustrates an operation foractivating two word lines simultaneously to apply a program currentICELL to a memory cell. Here, the program current ICELL is controlled bya bit-line voltage BL0 applied according to program information. In thecase of a reset operation for changing the resistance of the resistivestorage element to a high resistance, a pulse with large amplitude and ashort duration is applied to the bit-line. On the other hand, in thecase of a set operation for changing the resistance of the resistivestorage element to a low resistance, a pulse with small amplitude and arelatively long duration is applied to the bit-line. The operationdescribed in FIG. 2 is achieved according to these operations.Incidentally, since the phase change memory does not require theso-called erase operation such as that performed in a flash memory, itis possible to perform an operation for selectively applying a resetpulse or a set pulse according to storage information, as illustrated inFIG. 23A. The programming time can be reduced according to such anoperation.

FIG. 23B illustrates a modified example of the operation illustrated inFIG. 23A. The feature of the modified example lies in that aslowly-cooling pulse is applied to a bit line such as that described inNon-Patent Document 2. By extending a falling time of the pulse, it ismade possible to achieve an optimal crystallization temperature for eachcell so that resistance variation after ‘set’ can be suppressed.

FIG. 23C illustrates an example of another word line driving method. Afeature of this operation lies in that a set operation is performed bydriving word lines WL00 and WL01 in a two-stage manner. That is, in afirst period, the resistive storage element is once melted byapplication of a pulse with large amplitude. In a second periodsubsequent to the first period, the storage element is maintained in atemperature suitable for its crystallization by amplitude suppression.High speed of the set operation such as that described in Non-PatentDocument 3 can be realized by performing such an operation.

FIG. 23D illustrates an example of still another word line drivingmethod. A feature of this operation lies in raising the word lines WL00and WL01 at different timings. That is, in a first period, the storageelement is once melted even when any storage information is written byactivating two word lines WL00 and WL01. In a second period subsequentto the first period, by raising one word line WL00, the other word lineWL01 is held in the activated state. By such control, it is madepossible to suppress a cell driving current ICELL at the set operationto hold the temperature of the storage element in a temperature suitablefor crystallization. By controlling a cell current by a binary voltagedrive, an effect similar to the effect obtained by the operationillustrated in FIG. 23C can be obtained with a simple circuitconfiguration.

FIG. 23E illustrates an example of still another word line drivingmethod. A feature of this operation lies in a combination of the controlof the word line illustrated in FIG. 23D and the operation forperforming application of the slowly-cooling pulse to the bit line inthe second period illustrated in FIG. 23B. Both high speed of the setoperation and suppression of resistance variation after set can beachieved by performing such control.

FIG. 23F illustrates an example of an operation in which the bit lineBL0 is applied with a two-staged pulse such as that described inNon-Patent Document 3 instead of the slowly-cooling pulse. In this case,the cell driving current ICELL can be optimized like a slope pulse and aconfiguration of the driving circuit can be simplified by changing thebit line from analog driving method to a ternary driving.

According to the abovementioned configurations and operations, thefollowing effects can be obtained. That is, it is made possible toachieve a memory cell, in which a gate width of a select transistor islarge, with a small area by excluding a isolation area and forming 2T1Rcells as illustrated in FIG. 22. Further, as illustrated in FIG. 23, bycontrolling two word lines individually, it is made possible to realizecontrol of cell current by a binary driving to the word lines. Thereby,a set operation with a high speed and reduced resistance variation canbe realized with a simple circuit configuration. Incidentally, in theabove description, the example of the operation of controlling a voltageof a word line or a bit-line such that crystallization is performed oncea storage element is melted has been described. However, the principleof programming operation is not limited to the above-describedoperation, and various modified examples can be adopted. For example, aprogramming operation of, once raising the temperature of the storageelement to a temperature optimal for crystal core growth without meltingthe storage element, lowering the temperature of the storage element tosuch a temperature that crystal growth becomes optimal can be useddepending on the composition or the shape of the storage element. Inthis case, it is made possible to achieve a desired programmingoperation simply by controlling a program current using the programsystem of the present embodiment.

Ninth Embodiment

In an ninth embodiment of the present invention, a buffer cell structurein the phase change memory cell having the 2T1R configuration describedin the eighth embodiment will be described. A feature of the buffer celllies in that the contact on the resistive storage element is removedfrom the structure arranged around the memory array and the resistivestorage element and the bit line are disconnected from each other, asdescribed in the first embodiment.

FIG. 24 illustrates a memory-cell array TMCA and buffer-cell arraysYLTBCA and YRTBCA in a phase change memory according to the presentinvention. The memory-cell array TMCA is the same as that illustrated inFIG. 22, and an example including memory cells MC00 to MC77 arranged in8 rows and 8 columns is illustrated for simplification. The buffer-cellarrays YLTBCA and YRTBCA each includes buffer cells TBC0 to TBC7arranged in 1 row and 8 columns. Each of the buffer cells TBC0 to TBC7includes two transistors CT0 and CT1 and a resistive storage element RElike the memory cell. Further, connection of the resistive storageelement RE and the bit-line is cut, as described previously.

FIG. 25A illustrates a layout of memory cells MC00 to MC70 in the bitline BL0 and buffer cells TBC0 arranged at both ends of the bit lineBL0, FIG. 25B illustrates a cross-section structure of the same in across-section taken along AA′ in FIG. 25A and FIG. 25C illustrates across-section structure of the same in a cross-section taken along BB′in FIG. 25A. A feature of these cells lies in that they are formed onone active area pattern AA without formation of an insulator forisolation in a direction of a bit line. The resistive storage element(RL, 120) is arranged such that its longitudinal direction is parallelto a word line.

In the cross-section AA′, a resistive storage element within a memorycell is connected to the bit-line BL0 formed of a second metal layer(SM, 111) via a top contact (TC, 132), a first metal layer (FM, 110),and a first via (FV, 130). On the other hand, in a buffer cell TBC0 inthe buffer-cell arrays YLTBCA and YRTBCA, the contact on the resistivestorage element is removed and connection of the buffer cell TBC0 withthe bit line is cut.

In the cross-section BB′, a resistive storage element within a memorycell is connected to an active area (AA, 102) corresponding to a drainelectrode of transistors CT0 and CT1 via a bottom contact (BC, 131).Further, the active area (AA, 102) corresponding to a source electrodeof the transistors CT0 and CT1 are connected to a ground voltage supplyline formed of the first metal layer (FM, 110) via the bottom contact(BC, 131) and the top contact (TC, 132). A structure of the buffer cellTBC0 in the buffer-cell arrays YLTBCA and YRTBCA are similar to that ofthe memory cell.

By adopting such a configuration, the configuration in which the topcontact is excluded from the buffer cell is obtained, so that there isnot a concern about exposure or sublimation of the chalcogenide film 301at a top contact formation time, as described in the first embodiment.Thereby, it is made possible to prevent contamination of themanufacturing apparatus and production throughput is improved.

Incidentally, in FIG. 24, the configuration in which the buffer cellshave been arranged at both ends of the bit line has been illustrated forease of description. As described in the first embodiment, however,misalignment of the top contact hole or an area where the thickness ofthe Interlayer Dielectric film is uneven may also occur at both ends ofthe word line. In this case, exposure or sublimation of the chalcogenidefilm can be avoided by arranging buffer cells at both ends of the wordline like the bit-line. Further, electric characteristic variation dueto shape variation of a memory cell can be suppressed by furtherarranging a top contact outside the buffer cell, as described in FIGS.10A-10B in the second embodiment.

Tenth Embodiment

In a tenth embodiment of the present invention, another buffer cellstructure and another memory-cell array of the phase change memory cellwith the 2T1R configuration will be described. The feature of the tenthembodiment lies in that the resistive storage element RE has beenremoved from the buffer cell, as illustrated in FIGS. 11 to 13. FIG. 26illustrates another example of the buffer-cell arrays YLTBCA and YRTBCAin the phase change memory according to the tenth embodiment of thepresent invention. A memory cell array TMCA has a configuration similarto that illustrated in FIG. 24. The buffer-cell arrays YLTBCA and YRTBCAinclude buffer cells TBC0 to TBC7 including select transistors CT0 andCT1.

FIG. 27A illustrates a layout of memory cells MC00 to MC70 on a bit-lineBL0 and buffer cells TBC0 arranged at both ends of the bit-line BL0,FIG. 27B illustrates a cross-section structure in a cross-section takenalong AA′ in FIG. 27A, and FIG. 27C illustrates a cross-sectionstructure in a cross-section taken along BB′ in FIG. 27A. These cellsare formed in one active area AA like the structure illustrated in FIG.25. Further, a resistive storage element (RL, 120) is arranged in onlythe memory cells MC00 to MC70 such that its longitudinal direction isparallel to a word line.

In the cross-section AA′, a resistive storage element within a memorycell is connected to a bit-line BL0 formed of a second metal layer (SM,111) via a top contact (TC, 132), a first metal layer (FM, 110), and afirst via (FV, 130). On the other hand, in a buffer cell TBC0 in thebuffer-cell arrays YLTBCA and YRTBCA, the resistive storage element isremoved and a top contact (TC, 132) and the first via (FV, 130) areformed above and below the first metal layer (FM, 110) like the memorycell.

In the cross-section BB', a resistive storage element within a memorycell is connected to an active area (AA, 102) corresponding to a drainelectrode of the transistors CT0 and CT1 via a bottom contact (BC, 131).Further, the active area (AA, 102) corresponding to a source electrodeof the transistors CT0 and CT1 are connected to a ground voltage supplyline formed of the first metal layer (FM, 110) via the bottom contact(BC, 131) and the top contact (TC, 132). A structure of a buffer cellTBC0 in the buffer-cell arrays YLTBCA and YRTBCA is similar to that ofthe memory cell. By adopting such a configuration, an effect similar tothat of the eighth embodiment can be obtained.

Incidentally, in FIG. 26, the configuration in which the buffer cellsare arranged at both the ends of the bit-line has been illustrated forease of description. As described in the third embodiment, however,misalignment of the top contact hole or an area where the thickness ofthe Interlayer Dielectric film is uneven may also occur at both ends ofthe word line. In this case, exposure or sublimation of the chalcogenidefilm can be avoided by arranging buffer cells at both ends of the wordline like the bit-line. Further, electric characteristic variation dueto shape variation of a memory cell can be suppressed by furtherarranging a resistive storage element outside the buffer cell, asdescribed in FIGS. 10A-10B in the second embodiment.

In the foregoing, the invention made by the inventors of the presentinvention has been concretely described based on the embodiments;however, it is needless to say that the present invention is not limitedto the foregoing embodiments and various modifications and alterationscan be made within the scope of the present invention. Also, theabove-described first to tenth embodiments can be mutually combined inany way.

For example, a scale of the buffer cell can be changed according toperformance of a manufacturing apparatus. The configuration in whichcells are arranged two by two between the sense amplifier and the blockof word drivers has been described hereinabove, but a configuration inwhich cells are arranged one by one therebetween can be also adopted. Inthis case, a smaller memory-cell array can be achieved. On the otherhand, when misalignments of the top contact holes spread over a widerange or an Interlayer Dielectric film becomes thin over a wide range,exposure or sublimation of the chalcogenide film can be avoided byincreasing the number of buffer cells.

The embodiments of the present invention have hereinabove been describedusing the phase change memory where the resistive storage element hasthe chalcogenide film as the example, but the resistive storage elementis not limited to this configuration. For example, the present inventioncan also be applied to an MRAM or RRAM using a magnetic material andexposure or sublimation of the resistive storage element can be avoidedlike the phase change memory.

INDUSTRIAL APPLICABILITY

Since exposure or sublimation of a chalcogenide film can be avoided at aformation time of a top contact hole by providing an area where aresistive storage element or a structure where a top contact is removedbetween a memory-cell array and sense amplifier and a block of worddrivers, a phase change memory can be manufactured without contaminatinga manufacturing apparatus.

The invention claimed is:
 1. A semiconductor device comprising: aplurality of first wirings extending along a first direction; aplurality of second wirings extending along a second direction which isperpendicular to the first direction; and a plurality of non-volatilememory cells arranged at intersections of the first wirings and thesecond wirings, wherein each of the non-volatile memory cells includes aresistive storage element and a select transistor, and wherein, in thefirst direction, a most outer non-volatile memory cell of thenon-volatile memory cells is a buffer cell which does not have afunction to read and write data.
 2. A semiconductor device according toclaim 1, wherein: the resistive storage elements of the non-volatilememory cells except the most outer non-volatile memory cell areconnected to the first wirings through first plugs formed under thefirst wirings, no first plug is formed over the resistive storageelement of the most outer non-volatile memory cell, and the resistivestorage element of the most outer non-volatile memory cell is notconnected to any of the first wirings.
 3. A semiconductor deviceaccording to claim 2, wherein: a plurality of second plugs are formedover impurity regions of the select transistors and under the resistivestorage elements of the non-volatile memory cells, and the resistivestorage elements of the non-volatile memory cells are connected to theimpurity regions through the second plugs.
 4. A semiconductor deviceaccording to claim 3, wherein the resistive storage element of each ofthe non-volatile memory cells includes a chalcogenide film.
 5. Asemiconductor device according to claim 1, wherein the resistive storageelement of each of the non-volatile memory cells includes a chalcogenidefilm.